Hello from England. Currently trying to work out some ways to test if one of my transmissions may be slipping before removing it for servicing and writing off my season. Boat is a 2008 Velocity 390 with 525 EFIs (250 hours), Konrad drives and ace lowers (like shortys), Hydromotive 5 blades spinning out. Shifting on BAM super cyborg transmissions. Boat is an ex race boat with no cabin so very light at the front, I only run the saddle fuel tanks in the engine bay full of fuel, there are 2 tanks in the cabin but theyre empty.

3 weeks ago the boat was running strong in almost flat conditions, hit 82 mph with the trim pretty much level with hull, had a bit more throttle to go. When getting on plane the props break loose after 3000rpm at which point I back off so I dont over rev the engines, then everything bites and the boat is on plane and good to go.

This weekend I had a poker run and the conditions were sloppier, went out in the bay for a quick test before the run started and the boat wouldnt get on plane. STBD engine would not rev past 2k or 3k RPM depending on when I tried. Port engine would spin up but no matter what I did changing direction in the slop the boat just wouldnt hop up on plane. Abandoned run and came back in. Mechanic scanned both engines, no fault codes stored,100% available power, all readings normal, we then took the boat back out with computer plugged in and he found nothing. We believe both engines are strong. Current theory is that the engine that would happily rev up was possibly slipping in the transmission, the engine that wouldnt rev had no slip and was trying to get the boat on plane by itself in bad conditions and was being overloaded, I usually run a camera facing backwards and the rooster looked very anaemic like both props werent throwing water up compared to the last time out.

Have checked gear cables and the levers on the transmissions go fully into gear and stay in their detents,
Checked the props if they have hubs that may be spun, the hydromotives dont look to have hubs its all welded in and looks solid.
Drive oil was replaced between the trips out, no fur on magnets indicating issues there.
Checked the oil on the dipstick in the transmission, its still bright red. Although havnt yet drained any oil from the bottom where there may be some signs of shrapnel etc...
I read on the BAM website there is a procedure to purge the air out of the gearbox then check a different line on the dipstick called 'hot fill', this is next on our list.
Ran on the hose on trailer when flushing and in neutral the props move very slowly, probably about 3 rpm. I understand if this was much worse the clutch plates could be damaged or warped?

Are there any other signs to look for or tests we can do to determine if we have problems? Maybe putting a temperature or pressure gauge on somewhere? Season will be over here soon and Ive managed 1 good day out this year due to work so hoping its something simple like it needs a bit more oil and if so I havnt damaged it if its ran low. 18 days until part 2 of our poker run. Thanks.

If the transmission is slipping you will know very quickly. After not more than a few seconds slipping under load the fluid will be turning brown and smell burnt.

If it still shifted OK as you were coming home I doubt if transmission slipping is the problem.

I think it's more likely your starb engine was weak for some reason. The fact that the props normally break loose is what makes me think that. Port broke loose like normal, starb wasn't producing any thrust.

I agree with Marc. Transmissions smell when they slip. I would run the boat on one engine at a time. See how many rpm's you can get with just one engine running. if you can only get 2000-3000 rpm on the stb motor and at what speed? then see what speed you can get with the port at 2000-3000 rpm. if running just the port, can you get to 5000 rpm? what speed are you at? you could also have a broken drive plate. The software for the scanner doesn't show you all the engine parameters and does it show ALL the sensor data? Check the basic health of the motor. pull plugs and compare them.

Thanks for your insights and ideas both. Mechanic scanned both engines and we took the boat out with the computer connected. Didn't pick up sensor faults at all. They are doing more digging in the next few days. After I did the purging procedure on the website I found the oil was only 3 notches up on the dipstick on port which I thought might be the one with the slipping and was also just a touch low on stbd so both need topping up. Nothing smells burned so that's sort of reassuring apart from the not knowing what the issue is. Will report back if we find anything. Thanks again 👍

Fuel volume/psi issues usually don’t get caught by marine ecu’s. Do fuel psi test first. Then compression.

Just to report back on this. Went through the engines again, no fuel or compression issues, no faults. Purged and corrected all the transmission fluid levels just to be sure not too low and not overfilled, now all is correct at hot fill which I checked several times throughout the day and verified next day that it sits on the cold fill on the dipstick, all good.

Ended up craning the boat in for a test and a friend was keeping an eye on it getting on plane. Looks like it may have been a combination of it being only 3rd time out on the boat, worst conditions I have ran it in and the high x dimension with NO weight up front that I wasnt fully used to. Tried to replicate the engine not reving and if Im not assertive enough with the throttles the props dont want to break loose, either one or the other depending what the sea is doing, direction etc....

The props break loose when adding power but I as chickening out a bit early and dropping the RPMs thinking something was wrong, also not getting on the trim fast enough. With watching back some videos was able to work out the technique properly for the boat. When they do break loose I only need to drop RPMs a little and give it a very short time and it will bite as I start giving it trim where it will hop a couple of times depending on the sea, once prop shafts are trimmed level with the hull it settles right down and takes off like a rocket.

So to take away from this I believe the problem was driver error on this occasion, mechanically and electrically the boat is solid. Bit more time behind the controls and Im starting to get used to its quirks. Next Il inspect the front fuel tanks and hoses to check theyre clean and safe enough to be used, if I can keep some fuel up front that will either help or change the dynamic again. Also when I wasnt sure what was happening I did a lot of research, the cost of rebuild kits has pushed me to be a lot more careful checking fluid levels from now on.

Thanks for everyones input on this, it gave me some good ideas of what to go through.